HOST(1)
NAME
host - look up host names using domain server
SYNOPSIS
host [-l] [-v] [-w] [-r] [-d] [-t querytype] [-a] host [
server ]
DESCRIPTION
Host looks for information about Internet hosts. It gets
this information from a set of interconnected servers that
are spread across the country. By default, it simply con-
verts between host names and Internet addresses. However
with the -t or -a options, it can be used to find all of
the information about this host that is maintained by the
domain server.
The arguments can be either host names or host numbers.
The program first attempts to interpret them as host num-
bers. If this fails, it will treat them as host names. A
host number consists of first decimal numbers separated by
dots, e.g. 128.6.4.194 A host name consists of names sepa-
rated by dots, e.g. topaz.rutgers.edu. Unless the name
ends in a dot, the local domain is automatically tacked on
the end. Thus a Rutgers user can say "host topaz", and it
will actually look up "topaz.rutgers.edu". If this fails,
the name is tried unchanged (in this case, "topaz"). This
same convention is used for mail and other network utili-
ties. The actual suffix to tack on the end is obtained by
looking at the results of a "hostname" call, and using
everything starting at the first dot. (See below for a
description of how to customize the host name lookup.)
The first argument is the host name you want to look up.
If this is a number, an "inverse query" is done, i.e. the
domain system looks in a separate set of databases used to
convert numbers to names.
The second argument is optional. It allows you to specify
a particular server to query. If you don't specify this
argument, the default server (normally the local machine)
is used.
If a name is specified, you may see output of three dif-
ferent kinds. Here is an example that shows all of them:
% host sun4
sun4.rutgers.edu is a nickname for ATHOS.RUTGERS.EDU
ATHOS.RUTGERS.EDU has address 128.6.5.46
ATHOS.RUTGERS.EDU has address 128.6.4.4
ATHOS.RUTGERS.EDU mail is handled by ARAMIS.RUTGERS.EDU
The user has typed the command "host sun4". The first
line indicates that the name "sun4.rutgers.edu" is actu-
ally a nickname. The official host name is "ATHOS.RUT-
GERS.EDU'. The next two lines show the address. If a
system has more than one network interface, there will be
a separate address for each. The last line indicates that
ATHOS.RUTGERS.EDU does not receive its own mail. Mail for
it is taken by ARAMIS.RUTGERS.EDU. There may be more than
one such line, since some systems have more than one other
system that will handle mail for them. Technically, every
system that can receive mail is supposed to have an entry
of this kind. If the system receives its own mail, there
should be an entry the mentions the system itself, for
example "XXX mail is handled by XXX". However many sys-
tems that receive their own mail do not bother to mention
that fact. If a system has a "mail is handled by" entry,
but no address, this indicates that it is not really part
of the Internet, but a system that is on the network will
forward mail to it. Systems on Usenet, Bitnet, and a num-
ber of other networks have entries of this kind.
There are a number of options that can be used before the
host name. Most of these options are meaningful only to
the staff who have to maintain the domain database.
The option -w causes host to wait forever for a response.
Normally it will time out after around a minute.
The option -v causes printout to be in a "verbose" format.
This is the official domain master file format, which is
documented in the man page for "named". Without this
option, output still follows this format in general terms,
but some attempt is made to make it more intelligible to
normal users. Without -v, "a", "mx", and "cname" records
are written out as "has address", "mail is handled by",
and "is a nickname for", and TTL and class fields are not
shown.
The option -r causes recursion to be turned off in the
request. This means that the name server will return only
data it has in its own database. It will not ask other
servers for more information.
The option -d turns on debugging. Network transactions
are shown in detail.
The option -t allows you to specify a particular type of
information to be looked up. The arguments are defined in
the man page for "named". Currently supported types are
a, ns, md, mf, cname, soa, mb, mg, mr, null, wks, ptr,
hinfo, minfo, mx, uinfo, uid, gid, unspec, and the wild-
card, which may be written as either "any" or "*". Types
must be given in lower case. Note that the default is to
look first for "a", and then "mx", except that if the ver-
bose option is turned on, the default is only "a".
The option -a (for "all") is equivalent to "-v -t any".
The option -l causes a listing of a complete domain. E.g.
host -l rutgers.edu
will give a listing of all hosts in the rutgers.edu
domain. The -t option is used to filter what information
is presented, as you would expect. The default is address
information, which also include PTR and NS records. The
command
host -l -v -t any rutgers.edu
will give a complete download of the zone data for rut-
gers.edu, in the official master file format. (However
the SOA record is listed twice, for arcane reasons.)
NOTE: -l is implemented by doing a complete zone transfer
and then filtering out the information the you have asked
for. This command should be used only if it is absolutely
necessary.
CUSTOMIZING HOST NAME LOOKUP
In general, if the name supplied by the user does not have
any dots in it, a default domain is appended to the end.
This domain can be defined in /etc/resolv.conf, but is
normally derived by taking the local hostname after its
first dot. The user can override this, and specify a dif-
ferent default domain, using the environment variable
LOCALDOMAIN. In addition, the user can supply his own
abbreviations for host names. They should be in a file
consisting of one line per abbreviation. Each line con-
tains an abbreviation, a space, and then the full host
name. This file must be pointed to by an environment
variable HOSTALIASES, which is the name of the file.
See Also
named (8)
BUGS
Unexpected effects can happen when you type a name that is
not part of the local domain. Please always keep in mind
the fact that the local domain name is tacked onto the end
of every name, unless it ends in a dot. Only if this
fails is the name used unchanged.
The -l option only tries the first name server listed for
the domain that you have requested. If this server is
dead, you may need to specify a server manually. E.g. to
get a listing of foo.edu, you could try "host -t ns
foo.edu" to get a list of all the name servers for
foo.edu, and then try "host -l foo.edu xxx" for all xxx on
the list of name servers, until you find one that works.